1. Field of the Invention
The present invention relates to a developing device and a process cartridge, and more particularly to a developing device using one-component toner for an electrophotographic image forming apparatus, and to a process cartridge including the developing device.
2. Discussion of the Background
A developing device using one-component toner is provided to an electrophotographic image forming apparatus such as a printer, copier, facsimile machine and the like. The developing device generally includes a toner layer regulating member and a developing roller for frictionally charging toner at a nip portion formed between the toner layer regulating member and the developing roller and forming a toner layer having a predetermined height.
The toner layer regulating member generally has two types: a blade member and a doctor roller.
The blade member is widely used in a conventional developing device. When the developing roller is a hard roller formed by metal or resin, the blade member is made of elastic material such as a urethane rubber. On the other hand, when the developing roller is made of elastic material such as rubber, the blade member is a metal blade and the like. Such developing device using one-component toner does not require developer carriers and a toner density sensor while a developing device using two-component toner does. Therefore, when the blade member is provided and a process linear velocity is relatively low, the developing device using one-component toner can be made at a relatively low cost.
However, in recent years, an image forming apparatus is required to increase a speed of image forming operations. That is, the developing device using one-component toner is also required to increase the process linear velocity and to extend an operating life. Since the blade member is held in contact with the developing roller by pressure, the surface of the developing roller is gradually worn away with age, and charge ability of the developing roller deteriorates. The deterioration of the charge ability reduces a toner charge volume so that background contamination may occur and small dot reproducibility may be decreased. Further, the toner accumulated at the nip portion of the developing roller may melt by frictional heat caused between the developing roller and the blade member. Then, the melted toner coagulates and forms a toner lump which may cause a white streak on an image. Therefore, it has seemed to be difficult to produce a developing device having a long-term durability.
To eliminate the drawbacks, the doctor roller is used instead of the blade member. The doctor roller remains stopped while the developing device is forming an image and is rotated with the developing roller while the developing device is not forming an image. This reduces toner coagulation at a nip portion and background contamination due to aging.
FIG. 1 shows structures of another developing device including the doctor roller. The developing device forms a developing device 1 including a developing roller 2, a doctor roller 3 serving as a toner layer regulating member for regulating a toner layer and applying a voltage to toner, a toner supplying roller 4, a toner conveyance member 5, a toner scraping member 6 for scraping toner adhering to a surface of the doctor roller 3, an inlet seal 7 provided in contact with an upper portion of the developing roller 2 for preventing the toner from blowing out from the developing device 1, and a toner hopper 8 serving as a case of the developing device 1 for preventing a leakage of the toner. The developing roller 2 of the developing device 1 is held in contact with a photoconductive belt 9 serving as a belt-shaped photoconductive element. The photoconductive belt 9 forms an electrostatic latent image on a surface thereof and moves in a direction A. The developing device 1 produces a full-color image through the following operations.
Toner is contained in the toner hopper 8. The toner conveyance member 5 rotates so that the toner is agitated and conveyed to the toner supplying roller 4. The toner supplying roller 4 is formed by a metal core and a foamed material such as polyurethane, silicon, EPDM, polycarbonate and the like. The toner supplying roller 4 is held in contact with the developing roller 2 and rotates clockwise or counterclockwise at a speed having a relative linear velocity difference with respect to the developing roller 2. The toner supplying roller 4 carries the toner and rubs it onto a surface of the developing roller 2 at a nip portion formed between the developing roller 2 and the toner supplying roller 4. The toner is preliminarily charged and is supplied to the developing roller 4 via a nip formed between the developing roller 2 and the doctor roller 3. The doctor roller 3 contacts the developing roller 2 by applying a pressure with a predetermined weight.
The toner supplied by the toner supplying roller 4 to the developing roller 2 passes through the nip between the developing roller 2 and the doctor roller 3 so that an amount of toner pass is regulated. This prevents the toner from being unevenly distributed to a thrust direction on the surface of the developing roller 2 and regulates to form a uniform toner layer on the surface thereof. Further, when the toner passes through the nip formed between the developing roller 2 and the doctor roller 3, the surfaces of the developing roller 2 and the doctor roller 3 are rubbed to cause a friction, and the toner is stably charged and is transferred onto a photoconductive belt 9. As a result, the electrostatic latent image is visualized and is conveyed for a next step of producing the full-color image.
When the toner is transferred onto the photoconductive belt 9, the photoconductive belt 9 travels in the direction A as shown in FIG. 1 and the developing roller 2 rotates in a direction B. That is, the developing roller 2 and the photoconductive belt 9 move in a same direction. Further, the developing roller 2 rotates at a speed 1.1 through 2.0 times faster than the speed of the photoconductive belt 9.
When the developing roller 2 is made of elastic material, the doctor roller 3 may be a hard roller, and vise versa. Generally, the doctor roller 3 employs an elastic base material such as a rubber. In that condition, when the developing roller 2 and the doctor roller 3 are left in contact without the toner between them for a long period of time, the rollers 2 and 3 may stick to each other and, as a result, a starting torque of the developing device may increase to cause damage or failure to a drive part such as a gear, a drive motor and the like.